Single-phase permanent split capacitor (PSC) motors are typically used in domestic appliances and air conditioner compressors. These motors have two windings usually denoted as a phase shift winding and a run winding. The phase shift winding has two capacitors, which are arranged in parallel so their capacitances add together, connected to it. One of these capacitors remains connected to the phase shift winding throughout operation of the motor, while the other capacitor is removed from the circuit once the motor reaches its operational speed. These capacitors are known as run and phase shift capacitors, respectively. The phase shift capacitor adds phase shift of the current through the run winding at low speed to increase the starting torque of the rotating field produced by the run winding to enable the rotor to commence rotation. The increased capacitance that helps generate the starting torque, however, does not optimize the performance of the motor once operational speed is reached. Therefore, the phase shift capacitor is removed from the circuit when the operational speed of the motor is reached.
The selective coupling of the phase shift capacitor into and out of the circuit supplying power to the motor windings is performed with different types of components. In some PSC motors, a mechanical switch is mounted to the output shaft of the motor with springs that bias the switch to a closed position, which couples the phase shift capacitor to the power circuit for the motor. As the output shaft approaches the operational speed of the motor, centripetal force acting on the springs stretches the springs and opens the switch, which decouples the phase shift capacitor from the circuit. As long as the output shaft rotates at a speed near the operational speed, the switch remains open and the capacitor remains out of the power circuit for the motor. Other PSC motors use relays to couple the phase shift capacitor selectively to the power circuit and still other PSC motors use sophisticated controllers that monitor the power circuit and remove the phase shift capacitor in response to predetermined conditions being detected in the circuit.
All of the previously known circuits for selectively decoupling a phase shift capacitor from the power circuit are relatively expensive. The mechanical switch is an additional component mounted to the output shaft, which not only is expensive to produce, but it also contributes to motor failure as the springs and the switch age and deteriorate. The relays are also costly items as are the microprocessors and other solid state control devices used to control the coupling and decoupling of the phase shift capacitor to the motor circuit. Retail prices for domestic appliances and compressors are closely related to the production costs for these devices. Reducing production costs in the motors used to run these devices would be beneficial.